Conventional rectifying circuit design converts alternating-current (AC) signals into a direct-current (DC) signal. Low-barrier Schottky diodes are often used in low AC power cases such as ambient RF signals. The AC-to-DC conversion bandwidth and efficiency are related to AC power level, diode characteristics, and load impedance. The diode characteristics also depend on frequency, input power level, and output current.
Since all diodes are not ideal with non-zero threshold voltage and non-linear I-V characteristics, the output of a rectifying circuit contains many harmonic signals in addition to the desired DC signal. Exemplary embodiments of the present invention relate generally to a novel harmonic harvesting circuit design for harvesting un-rectified AC power (contained in the fundamental and harmonic frequencies) at the output of conventional rectifying circuits. It is appreciated that the present invention can also be used with other types of conventional AC-to-DC converter circuits that have similar problems as the conventional rectifying circuits. As a result, this invention can improve rectifying efficiency as well as extend the upper frequency limit of conventional rectifying circuits, and thus enabling efficient energy harvesting from radio frequency (RF) signals at higher frequencies used for GSM (Global System for Mobile Communications) or Wi-Fi from 700 MHz to 2500 MHz.
One of the most important components in RF energy harvesting is a rectifier which converts RF signals into DC power. Low-barrier Schottky diodes are often used as switches in low-power, high-frequency rectifier circuitry due to their low turn-on voltage, small resistance, and small junction capacitance. However, typical conversion efficiency of ambient RF harvesters are less than 60% due to ohmic loss in diode resistance and un-rectified energy in the harmonics produced by the diode's non-linear I-V property and the rectifying process. (Un-rectified signals/energy are AC signals/energy that are not converted into a DC signal/energy by the rectifying circuit. Other types of AC-to-DC converter circuits may also be used with the present invention.) Rectifying efficiency higher than 50% is very difficult to achieve at low RF input power level as in the case of most ambient RF signals, and can only be achieved under very optimal conditions of impedance matching, input power level, frequency, and load impedance. The output harmonics has been shown in the literature to contain as much as 20% of energy which is wasted in conventional rectifier designs.
In addition to being able to harvest the RF energy contained in the harmonics produced by the non-linear property of rectifying diodes, the novel circuitry of the present invention can also improve the rectifying efficiency at the upper frequency limit of the rectifying diode where some portion of the input RF signal bypass the diode through its junction capacitance. This produces un-rectified RF signals at the fundamental frequency at the output of conventional rectifiers. Such RF signals are shorted to the ground by the large DC hold capacitor, and thus being wasted.
Disclosed herein is an exemplary harmonic harvester circuit design which converts un-rectified signals at the output of conventional rectifier circuits. In an alternative embodiment, a harvester circuit of the present invention connects between the output and input of a conventional rectifier circuit, thus forming a feedback loop circuitry. Additional embodiments of variations of harvester circuit topologies are comprised of different arrangement of rectifying diodes, filters, and impedance matching circuits.
In addition to the novel features and advantages mentioned above, other benefits will be readily apparent from the following descriptions of the drawings and exemplary embodiments.